The invention is based on a fastening element for a wiper arm, and on a method for producing it.
Fastening elements for windshield wipers, particularly for the wiper arm, often comprise zinc or a zinc alloy and are made by pressure casting a pressure casting mold. The zinc can be processed very well, is gentle to the pressure casting tool, and makes very good surface qualities possible. However, it has been found that zinc changes its microstructure in stressed regions over the course of time. The material becomes brittle, which increases the risk of breakage. This occurs to an increased extent in relatively long fastening elements because of the greater bending stresses and particularly at the connection points to adjoining components, such as in the region of a conical hub of the fastening element, which is pressed by means of a nut onto a knurled conical drive shaft of a windshield wiper drive mechanism. This creates tensile strains which are superimposed on the bending stresses, thus increasing the risk of breakage still further.
From European Patent Disclosure EP 0 584 018 A1, a fastening element with a sheet-metal part coated with plastic is known. The sheet-metal part has a conical hub on one end and a bearing point for a shaft on the other. In the region of the bearing point, the sheet-metal part protrudes with a curved portion out of the plastic, and in this portion there is a hole for suspending a tension spring. On its outer circumference in the region of the conical hub, the sheet-metal part has recesses or notches, so that a favorable form lock with be achieved between the plastic sheathing and the sheet-metal part.
In assembly, the fastening element is pressed directly with the conical hub of the sheet-metal part onto the drive shaft, which can damage the knurled structure of the drive shaft, so that the force transmission when a replacement part has to be installed may no longer suffice.
From German Patent Disclosure DE 44 44 328 A1, a fastening element of zinc, a zinc alloy, or aluminum or an aluminum alloy is known in which a reinforcing element is integrally cast at least in the region of a conical hub and surrounds the hub and extends in corelike fashion at least partway inside the body of the fastening element.
The reinforcing element has extensions that extend at least as far as the surface of the body of the fastening element. As a result, during production, the reinforcing element can be fixed exactly in its position, so that close tolerances can be adhered to in the critical hub region. DE 44 44 328 A1 also proposes that the reinforcing element extend over the entire length in the body of the fastening element, which makes economies of material and thus slender fastening elements possible.
The material bond between the casting material and the reinforcing element is reinforced by creating a form-locking bond with openings, holes or slits in the reinforcing element.
In order not to destroy the knurled structure on the drive shaft during assembly, it is proposed that the reinforcing element also be surrounded toward a center axis of the hub by cast material, preferably with a wall thickness of approximately 1.5 mm. As a result, the knurled structure can dig into the soft casting material so that strong forces can be transmitted with form- and force-locking. Because of the thin wall thickness, the bending stresses and tensile strains are absorbed virtually solely by the reinforcing element.
It is also known, particularly with relatively long fastening elements, to use aluminum or an aluminum or an aluminum alloy instead of zinc, to attain better long-term strength and to prevent breakage from brittleness. However, aluminum attacks the pressure casting tool, with attendant higher post machining costs and shorter service lives of the tool. The material costs for aluminum are also higher than for zinc.
The fastening elements described above from the prior art are cast or coated with a casting mold. If fastening elements of different lengths or different shapes are to be made, then a separate casting mold is required for each variant.
According to the invention, the fastening element comprises a middle, longitudinally extending, prefabricated carrier part, which is made from aluminum or preferably steel or a steel alloy in a bending and stamping operation on the ends of the prefabricated carrier part, fastening regions that are separated from one another later are cast in the form of heads, one of which has a conical hub and the other of which has a bearing point for the wiper arm. Zinc, a zinc alloy, aluminum, an aluminum alloy, magnesium, a magnesium alloy, or in certain embodiments plastic are particularly suitable as casting material for the heads.
In different variants of the fastening elements, often only the lengths and the shapes are different, while the fastening regions can be identical. With the fastening element of the invention, standard heads can be cast onto variously embodied carrier parts, thus creating different variants economically without requiring different casting molds. The number of tools required and thus tool costs are reduced. Another factor contributing to low tool costs is that the casting molds are smaller, since they do not surround the entire fastening element but instead only its ends, thus economizing on casting material. Production is also facilitated because the carrier part can be fixed precisely without needing extensions. The casting tools and a fixing device can be functionally and spatially separate and can thus be made economically.
With a flexurally rigid carrier part, for instance of steel, or a steel alloy and with a hollow or U- or T-shaped profile, and so forth, especially lightweight and slender fastening elements that present only little surface area to the wind can be created.
Further advantageous embodiments of the fastening element are possible with the characteristics recited in the dependent claims.
In the region of the hub, the fastening element is at especially high risk of breakage and twisting, because it is in this region that the greatest bending stresses occur, because the lever arms are longest. The bending stresses also have tensile strains superimposed on them that originate in the mounting of the conical hub on a conical drive shaft. It is proposed that in this region an inlay part that surrounds the head and is made of material with tensile strength be integrally cast with the head and in this region absorb the tensile and bending stresses and relieve the casting material. Steel, a steel alloy, aluminum, an aluminum alloy, or some other strong material familiar to one skilled in the art can be used as material for the inlay part. By this means, different demands of the casting material can be taken better into account, such as good processability, high surface quality, and so forth.
To attain a favorable flow of force from the head and especially the inlay part to the carrier part, in one feature of the invention the inlay part and the carrier part are joined together in form- and/or force-locking fashion. Peak stresses at the transitions from the carrier part to the cast-on head are avoided, and as a result the entire head can be made slender, lightweight, and with little casting material. Particularly in this feature, casting materials with low strength can be used, such as various plastics. To that end, the inlay part and the carrier part can be joined together with familiar plug- and clamp-type connections.
If the inlay part, in the region of the conical hub, comes into direct contact with the conically embodied drive shaft, the surface of the drive shaft can be damaged, especially if it has a knurled structure. To prevent this, the drive shaft can be hardened. In one feature of the invention, however, it is proposed that the inlay part be coated toward a center axis of the hub with casting material having a wall thickness of approximately 1.5 mm. The knurled structure on the drive shaft can dig into the soft material and can transmit high rotational forces by form locking without destroying the knurled structure. Mechanical stresses, such as pressure and bending stress, however, are absorbed virtually solely by the inlay part, because of the slight wall thickness.
The casting material toward the center axis of the hub can be injected through a second channel in the casting tool. However, it is advantageous if only one channel is required for both the inner and the outer casting material. According to the invention, this is attained with a region on a top and/or bottom face end of the hub by way of which the inner and outer casting material are joined to one another. In the production of the head, this casting material can flow from the outer part of the hub to the inner part of the hub, or vice versa, via this region. It is also proposed that recesses be made in the hub or that hubs with recesses be used, by way of which again the casting material flows, as a result of which the casting material can be distributed quickly and uniformly.
The fastening element is pressed by means of a nut with its conical hub from above onto a conical drive shaft. As a result, especially high assembly strains or pressures per unit of surface area and tensile stresses, which threaten the cross section, arise in the upper, narrower region of the hub, and especially on the surface of the head on which the nut is supported. In one embodiment of the invention, it is proposed that the inlay part have a collar toward the narrower region of the hub. Preferably, the collar extends to the surface of the head, and as a result the nut can be supported on the inlay part, while in the uppermost, narrowest region of the hub, the pressures per unit of surface area and tensile strains can be absorbed entirely by the collar.
A bearing point for the wiper arm is disposed in the second head of the fastening element. As a rule, the wiper arm is connected to the fastening element via a shaft which is guided in a bearing bush in the fastening element. A retaining clip is suspended from a pin in the fastening element by a tension spring which is connected to the wiper arm and braces the two components together and presses the windshield wiper against the windshield. Preferably, the fastening region is cast integrally into the head, which saves one assembly step later, but the shaft can also be supported directly in the casting material, if this material has adequate sliding properties, or if the shaft does not execute any, or only a few, relative motions in it.
The pin for the tension spring can also be cast integrally into the head, again saving a further assembly step. This effect is also attained with a carrier part which protrudes in the head region out of the casting material and has a hole or an undercut from which the tension spring can be suspended. The pins for the fastening elements are then omitted.
To reinforce the material bond between the casting material and the carrier part, it is proposed that the carrier part be form-lockingly joined to one or both heads by a shaping, for instance by means of apertures such as holes, slits, advantageously with collars or leadthroughs, by means of undercuts, notches, recesses and so forth. This is important especially with zinc or zinc alloys, which do not enter into any metal-to-metal bond with the carrier part in the pressure casting.
A method for producing the fastening elements is advantageously distinguished by the fact that with one pressure casting apparatus and two pressure casting molds, standard heads can be cast onto different carrier parts, by adjusting the pressure casting molds successively or simultaneously to one another or to a respective end of the carrier part in accordance with the variant to be made. The pressure casting molds can be small and can be made without fixing devices for the carrier part and thus can be made economically. Preferably in the method an adapter piece is disposed between the pressure casting molds; with it, the carrier part is fixed and the pressure casting molds are adjusted to one another. The adapter piece can be replaced quickly and easily for different variants, and furthermore the pressure casting molds can be fixed quickly relative to one another or to the carrier part.